Human exposure to airborne pathogens and secondary contaminants associated with germicidal UV lights disinfection
Open Access
- Author:
- Park, Seongjun
- Graduate Program:
- Architectural Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 08, 2024
- Committee Members:
- Julian Wang, Professor in Charge/Director of Graduate Studies
Bill Sitzabee, Outside Unit Member
Yuan Xuan, Outside Field Member
Donghyun Rim, Chair & Dissertation Advisor
Richard Mistrick, Major Field Member - Keywords:
- Indoor air quality
airborne transmission
UVGI
secondary organic aerosols
ventilation
ozone
photochemistry - Abstract:
- Since the outbreak of the novel coronavirus (COVID-19), the germicidal ultraviolet (GUV) system has emerged as a leading solution for effectively controlling airborne pathogens. Utilizing short-wavelength ultraviolet light (UVC), GUV serves as a disinfection technology targeting airborne microorganisms. Traditionally, UVC devices emitting light at a wavelength of 254 nm (GUV 254) are employed as upper-room systems, focusing on the upper portion of a room to avoid potential harm to human skin cells and eyes. However, there has been a recent surge in the application of UVC devices emitting light at a lower wavelength, specifically 222 nm (GUV 222). Unlike UVC at 254 nm, UVC at 222 nm can irradiate the entire occupied zone, because it possesses germicidal properties within the wavelength range of 207 nm to 222 nm possesses while posing no harm to human cells. GUV systems can effectively control the transmission of airborne pathogens. Nevertheless, it's essential to acknowledge that they might unintentionally introduce air contaminants such as O3 and secondary organic aerosols (SOA), which can adversely affect human health. Therefore, in the design and deployment of GUV systems, it is imperative to thoroughly evaluate the effects of operational and ventilation conditions on both its disinfection effectiveness and the potential production of secondary contaminants. Given this background, the overarching objective of this Ph.D. dissertation is to investigate the effects of GUV system operating conditions and ventilation conditions on disinfection efficiency and the inadvertent generation of air contaminants. Given the predominant use of computational fluid dynamics (CFD) for this study, the initial investigation focuses on evaluating the impacts of grid resolution, turbulence model, and particle modeling approach on simulating transport of indoor gases and particles in close proximity to a human body. Results reveal that Eulerian approaches exhibit improved alignment with experimental data when employing Large Eddy Simulation (LES) for the turbulence model. Meanwhile, the Lagrangian approach demonstrates the closest match with experimental data, achieving a concentration difference within the breathing box of less than 15% compared to experimental observations. However, both LES and the Lagrangian approach require high grid resolutions (y+ < 3.5) to ensure stable and dependable outcomes. Utilizing the quality control measures established through the initial study, the second investigation examines how ventilation and operating conditions of the upper-room GUV 254 system influence the disinfection performance. The results reveal that increasing the ventilation rate from 1.1 h-1 to 5 h-1 leads to approximately 85% airborne disinfection, while doubling the UV radiating volume results in a 60% disinfection rate. However, enhancing the UV fluence rate from 25 μW∙cm-2 to 50 μW∙cm-2 yields a relatively modest disinfection increase of 18%. Furthermore, the effect of room air mixing notably impacts the system's disinfection performance. The third investigation centers on modeling the formation and dispersion of oxidants and secondary contaminants generated by operating a GUV system utilizing UVC at wavelengths 254 nm and 222 nm. The results indicate that employing GUV 222 can lead to an approximate increase of 10 ppb in O3 concentration and 5.2 μg∙m-3 in SOA concentration compared to conditions without GUV. Conversely, GUV 254 raises SOA concentration by approximately 1.2 μg∙m-3, with minimal impact on O3 concentration. Furthermore, increasing the UV fluence rate of GUV 222 from 1 to 5 μW∙cm-2 results in up to an 80% increase in oxidants and SOA concentrations. Similarly, for GUV 254, raising the UV fluence rate from 30 to 50 μW∙cm-2 or doubling the radiating volume leads to up to a 50% increase in SOA concentration. The final investigation focuses on the one-hour exposure to airborne pathogens and secondary contaminants using six different positions of the GUV 222 system within an office environment. The findings indicate that the ceiling-mounted type offers the most substantial reduction in human exposure to airborne pathogens, achieving a decrease of 70 – 80%. Across all lamp positions, the O3 concentration in the breathing zone increases by 4 – 6 ppb after one hour of operation. However, the stand-alone configuration poses a potential risk of exposing occupants to elevated levels of O3 due to the creation of a high concentration zone (> 25 ppb) near the lamp. In summary, this Ph.D. dissertation provides valuable insights into optimizing the utilization of GUV systems to maximize disinfection effectiveness while minimizing adverse health effects on occupants. Furthermore, the study approach lays the groundwork for addressing uncertainties and errors inherent in studying the transport of airborne pathogens in indoor environments through CFD simulations.